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An Expert System for Solving Simple Cryptograms

An Expert System for Solving Simple Cryptograms. Carolyn Rucci Dr. Ralph Morelli 1 year project. Goal. Cryptanalysis Techniques. Cipher?. Encrypted text. Advantage: Perform a few quick, simple tests to determine cipher instead of having to try all ciphers.

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An Expert System for Solving Simple Cryptograms

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  1. An Expert System for Solving Simple Cryptograms Carolyn Rucci Dr. Ralph Morelli 1 year project

  2. Goal Cryptanalysis Techniques Cipher? Encrypted text Advantage: Perform a few quick, simple tests to determine cipher instead of having to try all ciphers.

  3. Analysis and decipherment of encrypted messages Background Information Cryptography Cryptanalysis The Art of Secret Writing. Analysis Decryption

  4. WWI Cryptographer What is an Expert System? Reasons about ciphers CipherExpert knowledge Makes informed, intelligent decisions

  5. Jess Expert System Cipher Expert • Prototype that uses a limited set of historical ciphers: Transposition, Caesar, Affine, Vigenere and Playfair Main Java Class Expert System Core Hcrypto Decryption Classes (Java) Hcrypto Analyzer classes (Java) Computer Science department libraries

  6. How Does the Expert System Work? Creates decision tree with ciphers Distribution of Letter Frequencies Index of Coincidence Used to determine where to go in tree Distribution based on an English text Measure of how flat the distribution is

  7. Easily Extensible Want to add new cipher to system: Create new rules distinguishing it from other ciphers Letter frequency distribution of new cipher

  8. Testing • Tested CipherExpert 36 times, using 3 texts of varying length

  9. Flow Chart Cipher Text • IC = Index of coincidence IC >= .058 false true Cipher dist = normal dist false IC<=.0455 true false true Cipher is Playfair Cipher is Vigenere Shift freq. = normal Cipher is transposition true Same freqs present Cipher is Caesar false true Error Cipher is Affine

  10. Goal • Use cryptanalysis techniques to determine which historical cipher was used to encrypt a text • Use that cipher to decrypt the text Advantage: More efficient and accurate than other methods. Easily Extensible

  11. Collaboration between a domain expert and an artificial intelligence specialist Rules are based on experts’ knowledge Inference Engine uses rules in knowledge base to “reason” Example: Doctor diagnosing a patient. What is an Expert System Patient response Sends answer Sends characteristics User Interface Knowledge Base User Inference Engine Sends possible result or asks for more data asks question about symptoms Sends question or diagnosis

  12. Summary of Findings NOTE: These values are for texts of ~2000 characters. For Vigenère and Playfair ciphers, keyword must be at least 4 letters, toensure these values.

  13. Cryptanalysis • Uses analytical techniques to decrypt a message when the key is unknown. • Letter Frequencies a b c d e f g h i j k l m n o p q r s t u v w x y z monographic frequency distribution of normal alphabet • Index of Coincidence thisisatestoftheindexofcoincidenceinmessages thisisatestoftheindexofcoincidenceinmessagesthi 5matches / 44letters ~ 11%

  14. Cipher Set Simple Substitution Ciphers  One cipher alphabet • Transposition—Flips the message • hello  olleh • Caesar—Shifts each letter by 3 • Plain alphabet: a b c d e f g h i j k l m n o p q r s t u v w x y z • Caesar alphabet d e f g h i j k l m n o p q r s t u v w x y z a b c • Affine • C = (aP + b) mod 26 • a & b integer contants, C = cipher letter, P = plain text letter • one-to-one relationship between letters

  15. Ciphers (cont.) • Vigenère • Many cipher alphabets created based on keyword • Length of keyword = # of alphabets • Cycle through alphabets when encrypting a message • Playfair – two letter pairs Case1: 2corners  replace w/other corners Case2: same row  replace w/letters to right Case3: same column  replace w/letters below

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